1. Field of the Invention
The present invention relates to a continuous process for the preparation of dialkyl carbonates by transesterification of ethylene carbonate or propylene carbonate with C.sub.1 -C.sub.4 -alcohols in the presence of a catalyst, the starting substances being passed in countercurrent to one another.
2. Description of the Related Art
It is know that ethylene glycol carbonate and propylene glycol carbonate (glycol carbonates) can be reacted with alcohols in the presence of catalysts to give dialkyl carbonates and ethylene glycol (EG) or propylene glycol. Although these reactions can proceed with a high selectivity, the processes have a number of disadvantages in their procedure. As a rule, the transesterification proceeds relatively slowly under normal pressure, so that the use of elevated temperature, often above the boiling point of the alcohol employed, is recommended, which results in the process being carried out in pressure vessels (German Offenlegungsschrift 2 740 243=EP 1082; and German Offenlegungsschrift 2 740 251=EP 1083).
In this process, the reaction usually proceeds only until equilibrium is established in the transesterification reaction. After removal from the pressure container, the reaction mixture must be removed from the catalyst very quickly, for example by flash distillation, so that the starting compounds are not reformed in a reversal of the formation reaction, for example when the alcohol of low boiling point distils off. During removal from the catalyst by distillation, the glycol carbonate still in equilibrium an decompose into carbon dioxide and into polyglycols and is therefore unavailable for further transesterification, the yield being reduced, and all the by-products mentioned interfere with the working up.
However, even if this removal of the catalyst is effected satisfactorily, further distillations must still be carried out. Thus, it is first necessary to separate high-boiling components (glycols and glycol carbonates) from the low-boiling components (alcohols and dialkyl carbonates). However, distillative purification of ethylene glycol, which must have a high purity for example for the preparation of polyesters, to remove incompletely reacted ethylene glycol carbonate, which in turn is preferably employed for the preparation of dialkyl carbonates, is not possible without restriction since both compounds form an azeotrope. A similar difficulty is found when methanol, which is the preferred alcoholic component, has to be separated off from the dimethyl carbonate (DMC) formed. These compounds also form an azeotrope, which can be distilled into its separate components only in a cumbersome manner (EP 894 and literature cited therein).